The present invention relates in general to voltage controlled tunable delay lines and more particularly to an apparatus for producing a control voltage for such a delay line in response to a digital control input.
Digital delay circuits are now commonly used to delay a digital signal by variable delay times. These delay circuits typically utilize a high stability reference clock to clock a programmable counter circuit. A typical digital delay circuit can produce a wide range of selectable delay times and the delay time produced is a highly linear function of the digital control data, thereby making the delay time easy to predict and control. However the resolution in delay time adjustment which can be obtained utilizing a digital delay circuit is limited by the period of the reference clock. With current semiconductor technology, the resolution limit is in the one nanosecond range. For higher resolution control over signal delay times, a delay line may be employed.
The delay time of a simple delay line can be adjusted with substantially unlimited resolution by trimming the length of the line. Delay lines can be simulated by lumped parameter networks wherein the delay time of the network is adjusted by adjusting the values of the network components. Such a delay line, described in U.S. Pat. No. 4,701,714, entitled TUNABLE DELAY LINE, issued Oct. 20, 1987, delays a signal by a variable delay time which can be controlled by varying the magnitude of an applied control voltage. The control voltage changes the capacitance of elements affecting the delay time of a lumped parameter network. The delay time resolution obtainable with this "tunable" delay line is much smaller than one nanosecond, depending on how finely the control voltage can be adjusted. While a digital to analog converter (DAC) could be used to produce the control voltage, the delay time produced by this tunable delay line is a somewhat nonlinear function of the control voltage and is therefore a nonlinear function of the digital input to the DAC. The nonlinear response of the tunable delay line makes the delay time produced by the delay line difficult to predict and difficult to control.
What is needed, and would be useful, is a circuit for converting digital control data to a control voltage for a nonlinear, voltage controlled delay line in such a way that the time delay produced by the delay circuit is a linear function of the digital control data. Such a circuit would simplify the use of the tunable delay line in conjunction with digital circuits.